There are known electromagnetic valves (e.g., electromagnetic hydraulic control valves) equipped with a linear solenoid that generate a drive output corresponding to an amount of transmitted current. For example, Japanese Unexamined Patent Publication No. 2004-144230 discloses such a valve.
The linear solenoid disclosed in Japanese Unexamined Patent Publication No. 2004-144230 includes a plunger that is slidably supported in a stator. The plunger is biased toward the bottom face of a yoke by a return spring. Thus, when current transmission through a coil is stopped, an end face of the plunger abuts against the bottom face of the yoke. However, when current is transmitted through the coil, the plunger moves away from the bottom face of the yoke against the biasing force of the spring.
When current transmission to the coil is stopped, the entire end face of the plunger abuts the bottom face of the yoke. As a result, when current begins to be transmitted through the coil, the area between the end face of the plunger and the bottom face of the yoke is under negative pressure, thereby hindering the initial movement of the plunger. Initial movement of the plunger is especially hindered when the space between the plunger and the yoke is filled with high viscosity oil (e.g., oil at low temperature, etc.).
In partial response to this problem, it is proposed to include a circular recessed portion J1 in the center of the bottom face of the yoke 34 so that only the outer circumferential edge of the plunger 32 abuts against the bottom face of the yoke 34 as illustrated in FIG. 1A. Thus, a space is ensured between the end face of the plunger 32 and the bottom face of the yoke 34. Also, a second recessed portion 43 (i.e., a breathing groove) extends radially through the annular abutted portion 44 so that oil can flow into and out of the space between the plunger 32 and the yoke 34. Thus, the plunger 32 moves more easily when current transmission begins.
When current transmission through the coil is stopped, the biasing member 5 biases the plunger 32 such that the plunger 32 abuts the bottom face of the yoke 34, as illustrated in FIG. 2A. However, when current transmission begins, magnetic flux I′ flows such that the stator magnetically attracts the plunger 32. In addition, magnetic flux II′ also flows in the abutted portion 44 between the end face of the plunger 32 and the bottom face of the yoke 34. Thus, the attractive force is generated that causes the attracting stator to magnetically attract the plunger 32. (This attractive force will be hereafter referred to as the first force indicated by the Roman numeral I.) In addition, the force by which the plunger 32 adheres to the bottom face of the yoke 34, i.e., attractive force in the direction opposite the first force I, is generated. (This force will be hereafter referred to as the second force indicated by the Roman numeral II.)
Solid line A in FIG. 2B graphically illustrates the behavior of the conventional valve. Broken line B represents a balance point of the plunger 32 where the plunger 32 is balanced in the axial direction of the stator (i.e., where the magnetic attractive force, the biasing force of the return spring, and the feedback axial force that acts on the spool are balanced).
The second force II can be substantially strong. As indicated by solid line A, the attractive force that acts on the plunger 32 is reduced especially on the side where the stroke of the plunger 32 is small. Also, two balance points 1, 2 are produced where the solid line A intersects the balance line B.
When current transmission to the coil begins and is gradually increased, the plunger 32 can jump instantaneously from the balance point 1 to the balance point 2. The spool moves integrally with the plunger 32.
Thus, in cases where the spool valve controls oil pressure, as illustrated by line C in FIG. 2C, the oil pressure P can jump from point 1 to point 2 as indicated by the arrow. This jump in oil pressure is undesirable.